This invention relates to a self-centering drill bit with a pilot tip and particularly relates to a twist drill formed with a pilot tip extending axially from a larger "fishtail" drill section having a substantially larger diameter than the pilot tip.
One of the most commonly used twist drills is formed with a chisel edge, or chisel point, at the work-engaging end of the drill. An example of a chisel edge drill is illustrated in U.S. Pat. No. 3,237,488 which issued to A.D. Parone et al. The chisel edge is formed normal to the axis of the drill and usually extends equally on opposite sides of the drill axis. While the chisel edge is the first portion of the drill to engage the workpiece, the cutting lips typically taper rearwardly and from the opposite ends of the chisel edge. In use, the chisel edge engages the workpiece and literally works the material in the immediate vicinity into an extrudate rather than forming chips for extraction. The worked material permits the drill to begin to move into the material of the workpiece whereby the cutting lips begin to cut the material and form removable chips.
While the chisel-edge twist drill is satisfactory for some drilling operations, it does not provide holes with accurately located centers or round holes. For example, the chisel edge drill, which includes two flutes and two cutting lips, tends to "skip" away from the desired hole location as the rotating chisel edge engages the workpiece. Further, any out-of-round characteristic of the drill or the chuck supporting the drill is transmitted to the working end during a hole-drilling operation. This results in the formation of an oversize hole. In addition, twist drills having a chisel edge typically have a relatively large web which is slightly less than the length of the chisel edge. Drills with larger webs require significant thrust in order to urge the drill into the workpiece. The larger webs also limit the effective space for chip removal through the flutes, the radial depth of which is determined by the web thickness.
In order to overcome some of the deficiencies encountered in the use of the chisel-edge twist drill, a variety of modified, special purpose tips have been developed in the past. For example, a split point tip was developed as a variation of the conventional chisel edge. One example of a split point drill is illustrated in U.S. Pat. No. 4,556,347 which issued to H.B. Barish. To form a split point, the faces of the drill tip which extend away from the conventional chisel edge are ground to form cutting lips coincidental with the location of the chisel edge and on opposite sides of the center of the chisel edge. This results in eliminating or reducing significantly the length of the chisel edge. It also provides additional cutting lips at the tip and effectively reduces the web thickness. By reducing the chisel length, less thrust is required. Also, by providing cutting lips near the tip of the drill and eliminating or reducing significantly the length of the chisel edge, chip formation occurs earlier in the drilling operation at the tip end to thereby more efficiently remove the material initially encountered by the tip. This action also assists in reducing the thrust required to urge the drill into the workpiece.
Due to the forward location of the cutting lips on a split point drill, material from the workpiece is removed promptly thereby allowing the forward end of the drill to seat firmly in the workpiece. This stabilizes the drill with the workpiece. In addition, due to the reduced length of the chisel edge, which approaches a point, any skipping action which may occur with a conventional chisel edge drill is avoided by a split-point drill and thereby provides a self-centering action.
Other types of drills or hole-forming bits are available which have a self-centering capability. These drills or bits have a variety of center-point tips which provide some degree of drill bit centering followed by the actual hole cutting facility. Some examples of these are spade bits, solid-center auger bits, power bore bits and brad-point bits. Typically, the tips of these bits are formed with cone or spike shaped tips, some of which are threaded. One such drill is illustrated in U.S. Pat. No. 2,332,295, which issued to J.J. Bouchal, wherein a mutifaceted tip terminates in a point at the forward end of the drill. This drill is particularly suited for drilling holes in wood. Another example is illustrated in U.S. Pat. No. 3,779,664, which issued to L.E. Caley et al. In this teaching, the drill is formed with a center tip in the shape of an inverted cone and is used to drill holes in non-ferrous metals and other low-yield strength materials.
In some of these hole-generating drills or tools, the cutting portions of the drill extend generally laterally, or only slightly angularly, from the center point. An example of drills of this type are illustrated in U.S. Pat. No. 2,600,286, which issued to C.H. Weiland, and U.S. Pat. No. 4,209,275 which issued to J.B. Kim. In use of such drills, the center point of the rotating drill enters the workpiece to establish the center of the hole to be drilled. Thereafter, the forwardly turned edges of the drill engage and scribe a circular cut in the workpiece which defines the size, or diameter, of the hole to be formed. As the drill continues to move into the workpiece, cutting lips between the center point and the turned edges cut away the intervening material which forms chips for removal through the flutes. Eventually, as the center point and the turned edges exit the other side of the workpiece, a disc-like section of workpiece material results between the center point and the turned edge in the completion of the forming of the hole. A cutting operation which results in the formation of the disc-like section, upon completion of the hole formation, is referred to as trepanning and drills or tools used in such operations are referred to as trepanners.
Other types of drills, referred to as step drills,. are formed with several sections or lengths in the axial direction. The forward section is formed with the smallest diameter, the last or trailing section is formed with the largest diameter and the intermediate section or sections are formed with intermediate diameters to form a stepped configuration. Step drills are typically used to form a variety of hole shapes such as countersink, counterbores, and so on.
Still another type of drill includes a pilot tip of small diameter which is similar to a step drill but with an exceptionally short tip. An example of this type of drill is illustrated in U.S. Pat. No. 3,592,555 which issued to B.A. Mackey, Sr. This drill is formed with relatively long cutting lips which include rounded portions. This requires significant increase in the thrust and torque requirements in the formation of a hole. While this drill shows a cutting point for stabilizing the drill upon penetration of soft as well as hard materials, the point is relatively flat and appears to be fragile. Also, the cutting point employs a chisel edge with its attendant disadvantages.
As disclosed in U.S. Pat. No. 4,565,473, which issued to Toshiaki Hosoi, a deep-hole drill is formed with a shank body and two separate tips which form the cutting portions of the drill. Each tip is brazed to the shank body and forms a straight cutting edge which extends inwardly of the drill from the periphery thereof. Each tip is further formed with a curved cutting edge which extends generally from the inward end of the straight cutting edge and terminates at the axial center of the drill. The tips are mounted independently on the shank body and are not provided with any backing support, for example by the shank body, in the area of the curved cutting edges. In addition, the drill is illustrated with straight flutes, although helical flutes could be formed as noted in the patent. Further, the illustrated embodiments include flute structure which has a flute width ratio of 0.5 to 0.02.
In one embodiment of the drill illustrated in U.S. Pat. No. 4,565,473, the tips are arranged in two diagonal quadrants, as viewed from the tip end of the drill, so that the straight cutting edges present a "fishtail" profile, as viewed from the side, with a two-piece pilot tip extending axially therefrom. In this arrangement, the rake faces of each of the tips overhang into the flute area of the drill and, thereby, do not present a continuous flute surface extending tot he tip of the working end of the drill. This overhang would appear to disrupt the smooth flow of chips from the working end of the drill into the flutes of the body. In addition, the tips are not formed integrally with the body of the drill but are brazed in place and thereby lack the inherent support attained with an integral structure. Further, the tips are generally flat in cross section and are located only in two diagonal quadrants of the tip end of the drill. In this arrangement, the tips do not obtain the type of backing support obtainable by an integrally formed, generally cylindrical pilot tip which includes tip portions located in all four quadrants of the tip end. This is particularly significant because the pilot tip portions of the tips illustrated in U.S. Pat. No. 4,565,473 extend axially beyond the end of the shank body and, consequently, have only the backing support provided by that portion of each tip.
While each of the drill bits described above possesses some merits in the drilling art, there is a need for a single drill bit which possesses many attributes such as a drill bit which is self centering, requires relatively low thrust, does not jam or form burrs at breakthrough, will not develop transient loading or wobble and thereby avoid breaking of corners, does not stall while drilling, will penetrate the workpiece with ease, is less susceptible to breakage, will produce a thicker chip for that dissipation from the cutting edge and powderless full waste removal, will generate a self feeding capability to enhance the lower thrust characteristic, will develop a burr-free accurately-formed round hole and will provide comparatively longer drill life.